Building With Style: Water-Shedding Details, Part II

Water-Shedding
Details, Part II
by Gordon Tully
One of the chief concerns in detailing
is to keep water from penetrating the
building through cracks and joints.
There are many ways to keep water
out of a joint. I will discuss some of
the most obvious: overlapping,
layering, sealing, overhanging, and
weeping.
Overlapping
Overlapping relies on the creation
of a dam high enough that water
cannot be driven over it by air
pressure. The principle is used in (for
example):
siding
roof shingles or tiles
most flashing
louvers
ridge vents
window and door sills
In principle, it is possible to
calculate the height of a safe overlap
for any application, but simple
calculation is defeated by other
complexities, such as:
1. The air pressure difference that a
joint or surface